Further supporting an implication of vitamin A signaling in PD is the observation that the sub-population of DA SNc neurons that is more prone to degenerate expresses the enzyme aldehyde dehydrogenase 1 subtype A1 (ALDH1A1) (8, 14–16), the synthesis enzyme of retinoic acid. The gene discussed is ALDH1A1; the disease is Parkinson disease.